Batch coil annealing furnaces (sometimes called "box annealing furnaces" or "bell shaped furnaces") are well known in the industry. Basically, such furnaces comprise a base upon which steel coils are stacked vertically, edge upon edge, and over which a removable inner cover is placed. An outer cover, in turn, is placed over the inner cover. The inner cover is removably sealed to the base. The outer cover contains some form of heat mechanism such as burners or radiant tubes which heat the inner cover which in turn radiates the heat to the work. For cooling, a cooling cover may be employed in place of the outer cover for cooling the inner cover which in turn functions as a heat sink for cooling the work. A fan arrangement within the base circulates a heat treat atmosphere about the coils.
Typically, the heat treat atmosphere within the inner cover has been inert, i.e., substantially nitrogen or, a reducing gas composition such as hydrogen-nitrogen has been used. Still more recently, substantially high hydrogen or high reducing atmospheres have been used as a furnace atmosphere in annealing metal strip for improved strip properties. Still more recently as disclosed in my prior application Ser. No. 051,702, a vacuum can be drawn within the inner cover for purge/cleansing purposes with a hydrogen gas backfill during the annealing process. The presence of hydrogen (and CO) significantly increases safety concerns should the seal arrangement between the inner cover and the furnace base leak trace amounts of oxygen into the inner cover during annealing or should appreciable amounts of the furnace atmosphere leak out and mix with air.
Initially, sand seals were used to seal the inner cover to the base. The sand seal is nothing more than a trough of sand into which the bottom edge of the cylindrical inner cover is placed. Sand seals are not acceptable should the furnace atmosphere contain any significant quantities of hydrogen.
Accordingly, the prior art has developed several elastomer seal arrangements to effect an improved sealing between the inner cover and the base. The seals are typically water cooled to prevent thermal deterioration of the seal. Examples of such arrangements can be found in U.S. Pat. Nos. Van Dyne 2,964,307; Blackman 3,411,763; Blackman 3,563,522; and Freund 5,006,064. All of these arrangements use the weight of the inner cover to compress the elastomer seal and in the process of compression effect sealing between the inner cover and the outside. Some arrangements also use mechanical clamps. Such elastomer seal arrangements are initially acceptable for slight, positive pressure application but eventually deteriorate and erode. They are not acceptable should a vacuum be drawn in the inner cover.
A conventional prior art elastomer seal applied to a batch coil annealing furnace is schematically illustrated in FIG. 1. In that arrangement, inner cover 1, insulated by Kaowool affixed to outer cover 3, rests on furnace base 4. Extending from inner cover 1 is a solid, annular elastomer seal 2 which seats on and is compressed by an annular ledge 11 extending from base 4. That is the weight of inner cover 1 compresses elastomer seal 2 against annular ledge 11 to make a seal. An insulating base trough 6 carries a water coolant connected through base couplings from a water supply in base 4. In addition, a trough 8 carrying water or oil is formed with annular ledge 11 in base 4 as shown and additionally acts as a liquid seal. This prior art arrangement is acceptable for slight positive pressure applications. It suffers from the defects discussed above which afflict other prior art elastomer seals. The weight of the inner cover results in an imperfect seal. The elastomer deteriorates and erodes in time. The liquid seal is not acceptable for vacuum applications.
Alternatively, liquid seals have been used for typical, slight positive pressure application. A double liquid prior art seal arrangement is disclosed in FIG. 1A. In that arrangement, inner cover 1 insulated by Kaowool affixed to outer cover 3 rests on furnace base 4. Extending from inner cover 1 is a first circular seal plate 5 which makes an imperfect sealing contact with an insulating material formed in an insulating base trough 6. Next, a second circular plate seal 7 forms a liquid seal with an oil trough 8. Finally, a third circular plate seal 9 forms a liquid seal with water trough 10. This seal arrangement is fairly effective for positive pressure systems but, of course, will not work should a vacuum be drawn within the inner cover. Also, there are problems inherent in any liquid seal arrangement should positive pressures within the inner cover significantly vary.
Finally, in my prior application Ser. No. 051,702, I disclose a unique water cooled elastomer seal arrangement which can basically be described as a pair of concentric circular O-rings with a vacuum drawn in the annular space between the O-rings to effect a seal. This is a more effective arrangement than the prior art discussed above since the vacuum and not the cover establishes the sealing force and should an oxygen leakage occur at the outer seal, the vacuum functions to draw the oxygen away from the system so that it does not inadvertently enter into the inner cover.
The elastomer seal must be protected against thermal and mechanical deterioration. Thermal protection is provided by cooling both flanges with water. Mechanical protection is more difficult. A lubricating grease is normally used to facilitate relative movement between flange sealing surfaces, preventing tearing of O-ring surfaces and compression. The grease attracts and retains dirt and debris and the dirt may cause leakage. If hot gases escape under pressure from the inner cover local face, temperatures of the seal can exceed safe levels and a catastrophic failure can occur. Should substantially pure hydrogen atmosphere be used, the inner cover may very well behave as a rocket. O-rings are, therefore, subject to premature failure. (This invention discloses a novel sealing arrangement that uses a dry diaphragm or membrane gasket rather than a greased O-ring. As such, it can replace all previously used seals of sand, ceramic, elastomer gaskets, and o-rings.)
Insofar as the specific application of liquid seals to the inner cover of batch coil annealing furnaces is concerned, there typically is provided high pressure, water cooling of the seal by means of a high pressure (in the area of 35-45 PSI) water line connected from the seal area (i.e., the inner cover) to the plant's water supply by means of a flexible metal or elastomer hose and, of course, a coupling connection. The coupling connection has to be made and broken whenever the inner cover is positioned on or removed from the stand. This means that the base and connection is subjected to heat and pressure which in turn can and does cause failures.
Insofar as the broader aspects of the invention are concerned, heat treat furnaces in general have doors with a variety of seal arrangements. Positive pressure heat treat furnaces have locking rings which cam the door into sealing engagement with the furnace enclosure. Vacuum furnaces typically use the vacuum drawn within the furnace to seal an elastomer seal extending above the door. The elastomer seals are water cooled. In general, the seals have to have carefully machined, water cooled surfaces between door and furnace enclosure to provide a positive seal.